Practical importance of information on storage, and dynamics of accumulation of snow on large territories and information of moisture content of soils is enormous. In this light, the main requirements imposed upon devices for measuring moisture content of soil and snow water storage are high accuracy of measurement, especially of soil moisture content measurement, and the possibility of continuous measurement so as to monitor dynamics of the process of variation of soil moisture content and snow water storage.
Widely known contact methods are most popular in measuring soil moisture content and snow water storage. Thus, snow scales for determining the geometrical thickness of snow pack and standard snow samplers that make it possible to sample snow and determine its density are used for measuring snow water storage. A weighing method is used for measuring moisture content of soil, which requires soil sampling and dehydrating. Standard burets are used for sampling, and samples are dehydrated in drying cabinets. These methods are labour consuming; they can be associated with a risk of life when meassurements are carried out in mountains, and cannot ensure the desired accuracy of measurements since, e.g. in measuring density of thawing snow, a part of the moisture is not measured by the densimeter. In addition, when soil moisture content is measured, it is impossible to obtain trustworthy results for large monitored areas using small-volume soil samples.
Methods using devices for remote measurement of soil moisture content and snow water storage are used concurrently with the contact methods. Such measurements enable automation of the measurement process and do not require sampling so that structure of the medium and processes occurring therein, such as snow accumulation, thawing and wetting are not affected. These measurements are based on recording electromagnetic radiations of both natural and artificial origin within a broad range of wavelengths from centimetric waves to gamma-radiation at the wavelength of 10.sup.-11 cm, including the areas of visible, ultraviolet and infrared radiation (E. V. Kolomeets, Sh. D. Fridman. Method for Determining Snow Water Storage and Soil Moisture Content By Detecting Cosmic Rays. (in Russian), 1981. Gidrometeoizdat Publishing House, Leningrad. p.3). A device for measuring moisture content of soil and snow water storage using penetrating electromagnetic radiation in a predetermined wavelength range comprises a detector of electromagnetic radiation connected to a recorder. Moisture content of soil and snow water storage are determined by a change in electromagnetic radiation which depends on soil moisture content and snow water storage values.
Aerial gamma-survey is most widely used for determining moisture content of soil and snow water storage (A. V. Dmitriev, Sh. D. Fridman. Fundamentals of Remote Methods for Measuring Snow Water Storage and Moisture Content of Soils Using Gamma-Radiation of the Earth (in Russian), 1979. Gidrometeoizdat Publishing House, Leningrad. pp.281; 288). A device for carrying out aerial gamma-survey comprises a detector of background gamma-radiation from the snow or soil surface mounted on board an aircraft which is used for repeatedly measuring gamma-radiation. Soil moisture content and snow water storage are evaluated by attenuation of gamma-radiation.
Devices for measuring soil moisture content and snow water storage making use of penetrating natural origin electromagnetic radiation cannot ensure simultaneous and independent measurement of soil moisture content and snow water storage, nor do they ensure the desired accuracy since soil humus and biomass of the vegetation layer influence the measurements.
Devices with artificial sources of penetrating radiation are used for measuring soil moisture content and snow water storage (E. V. Kolomeets, Sh.D. Fridman. Method for Determining Snow Water Storage and Soil Moisture Content Using Cosmic Rays (in Russian). 1981. Gidrometeoizdat Publishing House, Leningrad. p. 74). Thus, in a device for measuring moisture content of soil, use is made of an isotope neutron source, and a device for measuring snow water storage incorporates an isotope gamma-radiation source, and appropriate detectors are used in these devices. Moisture content of soil and snow water storage may be determined by the character of a change in the artificial radiation that passed through the body of snow or soil.
The employment of isotope sources results in certain difficulties in that it is necessary to provide biological protection, and it can bring about pollution of the environment. The greater the water storage being measured, the higher is the capacity of the isotope source; alternativly, the device should have several isotope sources and detectors placed at different altitudes. In addition, these devices cannot ensure the desired accuracy of measurement because of the influence of soil humus and biomass of a vegetation layer, especially in a wide measurement range, and one and the same device cannot be used for measuring both soil moisture content and snow water storage.
Known in the art is a device for measuring moisture content of soil and snow water storage, comprising a first detector of cosmic-ray neutrons that passed through the soil placed deep in the soil at a pre-set depth and a second detector of cosmic-ray neutrons placed above the soil at an altitude which is greater than maximum thickness of snow pack or height of vegetation layer and which is spaced from the first detector of cosmic-ray neutrons at a distance which does not exceed the path length of thermal neutrons before absorption in the atmosphere, each detector being electrically coupled to a respective series circuit including a unit for detecting pulses of a predetermined amplitude and a pulse counter having a control input to which is connected a timer having an output connected to a recorder (E.V. Kolomeets, Sh. D. Fridman. Method for Determining Snow Water Storage and Soil Moisture Content Using Cosmic Rays (in Russian), 1981, Gidrometeoizdat Publishing House, Leningrad. pp. 59;83;84).
This device makes it possible to carry out simultaneous measurement of soil moisture content and snow water storage, provided only that one of the measured values remains unchanged. Consequently, this device cannot ensure the desired accuracy of measurement as the values being measured change with time and are interlinked. Moreover, measurements accuracy is influenced by soil humus and biomass of a vegetation layer which are not taken into account during measurements.